This invention relates to a variable displacement compressor and, more particularly, to a variable displacement compressor with a compressor capacity control valve, which is suitable for rapid cooling of an automobile air conditioner.
In general, a conventional variable displacement compressor comprises a mechanism for converting rotating motion of a driving shaft into reciprocating motion of pistons of the compressor. The mechanism includes a rotating swash plate driven by the driving shaft and a wobble plate supported by the swash plate and connected to the pistons by connecting rods. The wobble plate is provided with a rotation preventing mechanism by which the wobble plate itself is prevented from rotating and provides only wobbling motion for reciprocating the pistons. The swash plate is pivotably supported by a driving member secured to the driving shaft so that it can change in inclination angle against the driving shaft. The change in the inclination angle causes change in piston stroke, whereby the compressor changes in capacity. The inclination angle of the swash plate is determined by pressure differential of refrigerant acting on both ends of respective pistons. The pressure differential is controlled by a compressor capacity control means not to unduly cool down the refrigerant, so that an evaporator will not be covered with frost.
A proposal of the compressor capacity control means is disclosed in U.S. Pat. No. 4,428,718 in which the compressor capacity or discharge volume is controlled in response to both the suction pressure and the discharge pressure thereof by increasing the pressure in the crank chamber through individual communication between the crank chamber and the suction cavity or the discharge cavity.
In this conventional method of controlling the compressor capacity, a discharged refrigerant is introduced into the crank chamber, so that an amount of blow-by gas substantially increases whereby the durability of the compressor is lowered because the lubrication oil in the crank chamber flows out into a refrigeration cycle without a suitable oil recovery means, and the compressor efficiency decreases. Further it is not taken into account that when atmospheric temperature decreases (to less than 5.degree. C.), the discharge pressure also decreases (to less than 3 kg/cm.sup.2 g), so that a crank chamber pressure necessary to control the compressor capacity can not be obtained.
A part of the refrigerant returning from the refrigeration cycle to the compressor leaks from gaps between the cylinders and the pistons and enters the crank chamber as blow-by gas. It is necessary to discharge the blow-by gas out of the crank chamber for one reason to maintain the pressure in the crank chamber at a desired value. In this case, the lubrication oil mixes with the blow-by gas and the discharge of the blow-by gas is accompanied by outflow of the lubrication oil in the crank chamber, so that the lubrication oil decreases and the durability of the compressor is lowered.
A method for recovering lubrication oil flowing out into the refrigeration cycle is disclosed in SAE Technical Paper Series No. 850040 (1985) in which the lubrication oil mixed with the blow-by gas flowing into the crank chamber through gaps between the pistons and the cylinders is separated from the blow-by gas and stored in the crank chamber to use again as lubrication oil. An amount of the recovered lubrication oil is proportional to an amount of the blow-by gas. From the view point of lubrication, it is desirable to increase an amount of the blow-by gas, but the increase of the blow-by gas amount results in lowering the compressor performance.
In the variable displacement compressor, the inclination angle of the wobble plate is changed by a pressure differential between the pressure in the crank chamber and the pressure inside the cylinder, so that in case thermal load in the refrigeration cycle is small, the pressure differential is small, and an amount of the blow-by gas decreases, which is followed by a decrease in lubrication oil returning into the crank chamber.
Further, it is necessary that the wobble plate connected to the pistons through the connecting rods move smoothly to incline against the driving shaft in order to effectively achieve the function of the compressor capacity control means and to smoothly reciprocating the pistons. The compressor capacity control means produces controlled pressure differential, and it is necessary that the wobble plate assumes a correct inclination angle in response to the pressure differential. Therefore, the wobble plate rotation preventing mechanism and a pivotal connection between the swash plate and the driving member must to move smoothly.